Recent studies have found that bone marrow-derived endothelial progenitor cells (EPCs) are present in[unreadable] the systemic circulation and home to sites of ischemic injury where they function to promote[unreadable] neovascularization. Cardiovascular diseases characterized by increased reactive oxygen species (ROS)[unreadable] stress and reduced NO bioavailability are associated with reductions in the number and functional activity[unreadable] of circulating EPC, and these reductions may diminish their angiogenic and reendothelialization capacity.[unreadable] EPCs share antigenic markers with hematopoietic stem cells and differentiate into the endothelial lineage[unreadable] in vitro. A number of cytokines, growth factors and adhesion molecules have been implicated in the[unreadable] mobilization of EPCs from the bone marrow stem cell niche and in their homing and differentiation at sites[unreadable] of vascular damage. However, the signaling and transcript!onal regulatory mechanisms that control EPC[unreadable] behavior are incompletely understood. The PI3K/Akt signaling pathway is an important regulator of growth[unreadable] responses in a number of systems, and some evidence has implicated this signaling pathway in promoting[unreadable] the proliferation and differentiation of EPCs under conditions of basal ROS-mediated signaling and[unreadable] compensated oxidant stress. PI3K/Akt signaling contributes to the regulation of the FOXO transcription[unreadable] factors that play a role in cellular resistance to oxidant stress. The proposed studies will test the[unreadable] hypothesis that FOXO transcription factors control EPC proliferation and differentiation and regulate their[unreadable] resistance to oxidant stress. To achieve these objectives, we will first characterize the FOXO isoforms in[unreadable] EPCs with respect to their expression, regulation by ROS and subcellular localization (Aim 1). We will then[unreadable] use gain-of-function and loss-of-function strategies to determine the role of FOXO isoforms in controlling[unreadable] EPC function in vitro including proliferation, differentiation, migration, apoptosis and resistance to oxidant[unreadable] stress (Aim 2). Finally, we will construct lines of transgenic mice that conditionally express constitutivelyactive[unreadable] and dominant-negative FOXO isoforms in EPC and examine the implications for EPC behavior and[unreadable] response to ischemic injury (Aim 3). These studies should provide mechanistic information on the FOXO-mediated[unreadable] regulatory pathways that control EPC phenotype and the relationships between redox stress and[unreadable] EPC function.